The present application is directed generally to a voltage regulation and modulation circuit of a contactless device in a contactless communication system.
FIG. 7 illustrates a typical contactless system 700 having contactless reader 710 and contactless card 720. Reader 710, also known as a PCD, includes an antenna 712 electrically coupled to an electronic circuit (not shown). Contactless card 720, also known as a smart card, a tag, a PICC, or an RFID card, has inductive antenna 721 and circuitry and microcontroller 740 coupled to inductive antenna 721.
In operation, when contactless card 720 penetrates a transmission field of reader 710, reader antenna 712 transmits to contactless card 720 a carrier signal, which generates a transmission field to supply contactless card 720 with power and data. The transmission field induces a voltage in card antenna 721, and this induced voltage is tuned by tuning capacitor 722 to generate input voltage Vin. In return, contactless card 720 is capable of transmitting data by load modulating the carrier signal. This load modulated signal is detected by reader antenna 712. The communication between the reader and the contactless card may be defined for example by ISO (International Organization for Standardization) 14443, Type A/B/C, 15693, 18000, etc.
The amplitude of induced input voltage Vin in antenna 721 of contactless card 720 experiences significant variations as the distance and orientation of contactless card 720 change with respect to reader 710. In order to protect contactless card 720 from excessive voltages and to support the communication, i.e., modulation/demodulation, between reader 710 and card 720, a regulation of input voltage Vin is necessary. Once input voltage Vin is regulated, modulation and demodulation can be performed.
In addition, since microcontroller 740 embedded in contactless card 720 shows an impulsive current consumption profile during operation, proper spike suppression measures are necessary in order to avoid communication errors during the phase in which reader 710 is in reception mode, but microcontroller 740 is still operating.
As shown, contactless card 720 has a voltage regulator with an envelope detector which follows the amplitude of input voltage Vin. The output of the envelope is compared with a reference voltage KVref and the resulting error signal controls a shunt transistor current, Ishunt-2, of transistor 733. An additional transistor current, Imod, of transistor 736, is used for load modulation and its gate voltage, Vmod, must be adjusted based on input voltage Vin to control the modulation depth. The needed spike suppression is obtained using constant current source 731 to supply microcontroller 740 with a constant supply current, Isup, whose value must be adjusted according to the strength of the transmission field. Parallel regulator 735 fixes supply voltage VDD by shunting any excess supply current Ishunt-2.
The voltage regulation provided by contactless card 720, however, has numerous disadvantages. For example, it is costly in terms of area because it requires field shunt 735, current source 731, modulation transistor 736, and rectifiers, that is diodes 723-728 and capacitances 729, 730. Further, by using a variable current source 733 as a shunting device, the transmitted modulation depth is amplified, effectively distorting the transmission field and requiring additional circuitry to compensate for this effect during reception.
Moreover, field shunt control 734 and supply current Isup control 732 are not independent. Rather, supply current Isup must be adjusted according to the transmission field strength, such as by sensing the shunt current Ishunt-1 and increasing supply current Isup as long as shunt current Ishunt-1 is above a predetermined threshold. Further, in order to keep supply current Isup constant, a large capacitance Csup 729 is needed and must be adjusted according to the transmission field strength.
In a weak transmission field, current source 731 must be switched off during a modulation pulse, known as edge-boosting, in order to decrease the input voltage Vin rising time, that is the side-bands, and switched on again as soon as input voltage Vin reaches the regulation level. This causes overshoots which must be suppressed by keeping field shunt 735 active during communication between reader 710 and contactless card 720.
Finally, using field shunt 735 in parallel with microcontroller 740 requires driving the voltage Vshunt at the base of shunt transistor 733 to zero during startup, to assure that contactless card 720 starts in every transmission field condition. As soon as Vin increases, field shunt 735 must be fast enough to limit its value. This causes an unavoidable overshoot in input voltage Vin, and thus requires an additional shunt control circuit 734 at startup, based on a rough reference voltage since Vref is not yet available.